The present invention relates to an optical fiber cable, and more particularly, an optical fiber cable having a core with a bore in which at least one optical fiber is loosely contained and including a single strength member unit in an outer jacket around the core which provides both tensile strength and an aerial suspension means. The cable has a neutral surface associated with bending of the cable in a plane of maximum bending energy which is located within the outer jacket and outside the core bore.
Optical fiber is now used in a variety of telecommunication applications because of its small physical size and high bandwidth capacity. An optical fiber cable typically contains many optical fibers. The optical fibers can be contained in the cable in a variety of configurations, such as, for example, in an optical fiber ribbon, as a fiber strand or loosely enclosed in a buffer tube.
An optical fiber is a mechanically fragile structure. The optical signal transmission characteristics of an optical fiber can substantially degrade if the fiber is mechanically stressed. If a fiber is too severely mechanically stressed, the fiber can become non-functional for purposes of optical signal transmission in a telecommunication application.
It is not uncommon that an optical fiber cable containing an optical fiber or optical fibers will undergo handling or be exposed to a physical environment that can stress the fiber or fibers within the cable. For example, an optical fiber contained in an optical fiber cable can experience stress and strain when the cable is bent or stretched during winding on a reel for purposes of storage, or during or after installation along and over another surface, in a pipe or duct or suspended in air from vertical supports. Also, the fiber in a cable can be mechanically stressed if it is pinched between other cable components and because of the difference between the coefficients of thermal expansion for the optical fiber and the other components in the optical fiber cable containing the fiber.
When an optical fiber cable is bent, bending occurs along a neutral surface plane which is associated with the cable bending and extends along the longitudinal length of the cable. The intersection of the neutral surface plane with a cross-section of the cable is a neutral axis.
If an optical fiber cable is of uniform construction in all directions radially of its axis, the cable has the same rigidity, e.g., resistance to bending, in all directions transverse to the axis. However, if there are discrete components, such as strength members in portions of the cable, there are two preferred directions of bending in a preferred plane of bending or there may be more than one direction of bending in which the cable can be bent more easily than in other directions. Thus, there can be a longitudinal plane (MIN-BP) intersecting the cable in which minimum bending energy is required to bend the cable. As viewed in cross-section, a neutral axis called xe2x80x9cNAMinxe2x80x9d is associated with the bending of the cable in the MIN-BP, and NAMin is perpendicular to the MIN-BP and may intersect the cable axis. With such structure there is another such plane (MAX-BP) in which maximum bending energy is required to bend the cable, and there is a similar neutral axis called xe2x80x9cNAMaxxe2x80x9d which is associated with the bending of the cable in the MAX-BP and is perpendicular to the MAX-BP but may not intersect the geometric center of the cable.
When a radially non-uniform optical fiber cable is subjected to bending forces, the cable will seek to orient and twist itself to cause bending to occur in the plane for which a minimum of energy is required to bend the cable, i.e., the MIN-BP. When a cable is bent in a particular plane, the material of the cable at opposite sides of the neutral surface plane associated with the plane of bending is respectively compressed and concave, and stretched and convex and in tension. During bending of the cable, any component in the cable which is free to shift radially of the cable, such as an optical fiber loosely received in the bore of the core, tends to migrate from the portion of the cable under tension or compression to the portion where strain is minimized. The cross-sectional area in the cable within which any loosely held fiber can move, the length of the fiber in relation to the cable and the plane in which the cable is bent determine the location in the cable where the fiber will become positioned as a result of the bending of the cable. If, during bending of a cable, a loosely held fiber within the cable becomes positioned away from the neutral surface associated with the bending of the cable, elongation or contraction stress can be applied to the optical fiber if other expedients are not employed. Although it is desirable that any loosely held fiber in the cable is positioned on or near the neutral surface associated with the expected bending of the cable, which in many circumstances will be in the MIN-BP, it is possible to reduce or eliminate such stress by suitably selecting the size of the bore and the excess of the fiber length with respect to the rectilinear length of the bore axis (EFL).
Prior art optical fiber cables have been designed to include features which control the behavior of the cable when subjected to bending and control the location of the neutral surface in bending to limit the stress on fibers in the cable. For example, the optical fiber cable of U.S. Pat. No. 4,844,575, incorporated by reference herein, includes two diametrically opposing strength members in the cable jacket to provide the cable with a MIN-BP having an associated neutral surface which intersects the center of the cable and the centers of the opposing strength members. Such cable can be bent most easily in either of two directions.
In addition, U.S. Pat. No. 4,836,639, incorporated by reference herein, discusses the problems of winding and unwinding a pipe or tube containing optical fibers around a drum and discloses an optical fiber cable which includes one or more strength members within the tube wall and optical fibers which assume positions at the inner wall of the jacket of the cable. The strength member(s) of the cable and the tube wall of the ""639 patent position the neutral surface associated with bending of the cable in the MIN-BP near or coextensive with the position of the fibers in the cable with bending and so that when the tube is wound on a drum, the strength member or members are nearer the drum axis than the fibers, i.e., radially inwardly of the fiber. While the solution of the ""639 patent can be useful when the tube is wound on a drum, the solution is not satisfactory when the tube is used in other applications, e.g., aerial applications, or when the optical fibers are within a core comprising elements, such as a buffer tube, strength members, armoring, etc., which is surrounded by the tube of the ""639 patent as an outer jacket. Thus, in aerial applications the strength member is above the optical fibers, the loose optical fibers do not move significantly toward the strength member or the neutral axis described in the ""639 patent.
Although the ""639 patent indicates that only one reinforcing wire can be used, the ""639 patent also indicates that the number of reinforcing wires should be greater than one in order to insure that the cable is wound around a drum in the intended direction. In fact, if only one reinforcing wire were used, the patent does not indicate how a preferred direction of bending would be obtained.
The inclusion multiple strength members within a cable jacket can be disadvantageous for several reasons. First, the arrangement of a plurality of strength members in the cable jacket can make the cable extremely stiff. An overly stiff cable makes handling and maneuverability of the cable difficult because substantial energy would be required to bend the cable in a plane other than the MIN-BP with a minimum of twisting, which often is desirable and required during and after installation of the cable. Also, the inclusion of multiple strength members in the jacket greatly can increase the cable weight and the size of the cable in diameter and bulk to cause other undesirable inefficiencies. Further, the manufacturing step of extruding plastic over multiple strength members to obtain a desired jacket structure can be complex and difficult. Finally, it can be more difficult to secure aerial hardware to multiple strength members than to a single strength member in an aerial installation of a cable.
There are prior art cables suitable for aerial installation, see, for example, U.S. Pat. Nos. 4,097,119 and 5,095,176, incorporated by reference herein, which include metallic messenger wires which are connected to the main body of the cable by a thin web of jacket material and which can be used to suspend the cable securely from vertical supports. In this cable design, an additional longitudinal strength member, such as a reinforced rod or a metal sheath bonded to the jacket, is required in the core of the cable, because the messenger wires are not sufficiently coupled to the layers around the optical fiber to provide the cable itself with sufficient pulling and anti-compression resistance to minimize stress on the fibers in the aerial installation of the cable. In other words, the messenger wires do not provide a dual function of cable suspension and stress resistance. Also, the inclusion of strength members in the core or bonding of a metal sheath to the jacket can make the cable undesirably stiff.
Other optical fiber cables suitable for aerial installation, see, U.S. Pat. Nos. 5,125,063 and 5,448,670, incorporated by reference herein, include two diametrically opposed strength members embedded in a jacket which encloses a central tube loosely surrounding optical fibers. In an aerial installation, these cables are either clamped directly to a vertical support, or to a separate and independent messenger wire which extends along a series of vertical supports and which connects to and carries the weight of the installed cable. Such a cable design is inefficient because two strength members are required and because of the disadvantages described hereinbefore.
Similarly, the optical cable suitable for aerial installation described in U.S. Pat. No. 4,798,443, incorporated by reference herein, which includes a plurality of non-metallic reinforcing members embedded in the jacket and extending generally parallel to the axis of the cable, and which cable can be clamped directly to the vertical supports in an aerial installation, has some of the same disadvantages associated with the cables of the ""670 and ""063 patents. Although the ""443 cable design provides for a plurality of optical elements to minimize strain on the fibers in installation, where each optical element comprises several buffer tubes loosely carrying individual fibers and disposed around a non-metallic central member, this design may be more difficult and expensive to manufacture and access to the fibers at midspan of the cable is also more difficult.
Therefore, there exists a need for an optical fiber cable which is compact, has a small diameter, is lightweight, efficiently protects fibers loosely contained therein from mechanical stress in an aerial installation of the cable, which not only provides a preferred bending plane, i.e., the MIN-BP, but also allows for relative ease of bending of the cable in a plane other than the MIN-BP as compared to prior art cables and provides preferred directions of bending with respect to the MAX-BP.
In accordance with the present invention, an optical fiber cable includes a single strength member unit comprising a single strength member or a plurality of strength members within, and preferably embedded in, an outer jacket which encircles at least one loosely held optical fiber.
Preferably, the jacket encircles a core with at least a buffer tube having a bore in which a plurality of optical fibers are loosely contained, i.e., the cross-sectional area of the bore is greater than the cross-sectional area of the fiber or fibers. Also, preferably, the fibers have excess fiber length (EFL). The size of the bore and the EFL are selected so that the optical fibers are not stressed by any normal forces not absorbed by the strength member(s) in the single strength member unit.
Preferably, the outer surface of the jacket conforms to the surface of a cylinder and the outer surface of the core also conforms to the surface of a cylinder but the axis of the core is displaced with respect to the axis of the jacket in the direction away from the single strength member unit with the axis of the core and the axis of the jacket in the same plane. Preferably, the longitudinal axis of the single strength member unit also is in said same plane.
A plane of minimum bending energy (MIN-BP) for the cable is defined mainly by the physical properties and position of the single strength member unit but is also affected by the cross-sectional shaping of the jacket. Such shaping and the physical properties and position of the single strength member unit also define a plane of maximum bending energy (MAX-BP) perpendicular to the MIN-BP for the cable. The properties and position of the single strength member unit and the shaping of the outer jacket are selected so that the neutral surface plane associated with bending in the MIN-BP is the same as the plane in which the axes of the single strength member unit, the jacket and the bore lie.
In a preferred embodiment, the neutral surface plane associated with bending of the cable in the MIN-BP intersects the bore of the core and the centroid of the single strength member unit. The neutral surface associated with bending of the cable in the MAX-BP is within the jacket and outside of the bore of the core. However, the cable can be bent with less force than in the prior art in planes other than the MIN-BP, including the MAX-BP, and the single strength member unit provides the cable with tensile stiffness and antibuckling properties.
In a further preferred embodiment, the physical properties and position of the single strength member unit in the outer jacket and the shaping of the outer jacket provide that the neutral surface associated with bending of the cable in the MIN-BP intersects the geometric center of the cable and the geometric center of the single strength member unit. Further, the neutral surface associated with bending of the cable in the MAX-BP is orthogonal to the neutral surface associated with the MIN-BP. With such structure, the optical fiber, or the optical fibers, are at or close to a neutral axis (NAMin) when the cable is bent in the MIN-BP and bending of the cable in other directions encounters greater resistance.
The at least one loosely held fiber has an EFL, and the EFL and the ratio of the bore cross-sectional area to the cross-sectional area of the fiber or fibers are selected so that stress on the fiber or fibers is minimized when the cable is bent in the MAX-BP.
In a preferred embodiment, the core of the cable itself constitutes an independent and self-contained optical fiber cable which can be used without the outer jacket. The core preferably includes a central buffer tube with a bore and the tube loosely holds optical fiber ribbons in a stack arrangement.
In a further embodiment, the core in the cable includes at least one strength layer, such as an armor layer, one or more rigid dielectric strength members or a reinforced aramid yarn layer. The strength layer is disposed between the buffer tube and a core jacket. The core jacket is encircled by the outer jacket with the embedded strength member, which provides tensile stiffness and antibuckling properties to the core and to the cable while allowing the cable to be bent in the MAX-BP more easily than in the prior art.
In still another embodiment, a release coating layer is disposed between the inner surface of the outer jacket of the cable and the outer surface of a core jacket. The release layer releasably couples the outer jacket of the cable to the core, which includes the loose fibers, to provide for easy access to the fibers within the core.
In another aspect of the invention, the cable with the single strength member unit in the jacket is adapted to be installed aerially. In an aerial installation, portions of the single strength member unit within the outer jacket are exposed at intervals along the length of the cable and the exposed portions are secured to respective vertical supports. The single strength member unit in such aerial installation of the cable can support the entire weight of the cable and provide the core and the cable with tensile strength and antibuckling properties between the exposed portions while permitting the cable to bend with relative ease in the direction of the suspension curve between the supports.